So you took the recipe for Penne with Parmesan Cream and Prosciutto home only to realize you didn't have any heavy cream. You probably thought, great I'll just make this some other time. But wait! You can use a simple culinary substitution to ensure that your dish will still turn out.
To substitute 1 cup of heavy cream, mix 3/4 cup of milk and 1/3 cup of butter/margarine (melted and cooled). Just don't try to whip this, it won't work. Many people insist that it can only be substituted with whole milk (as opposed to skim or non-fat), however I say in a pinch, give it a try and see what you end up with - but if it's for an important feast, you may just want to run to the store.